48 BULLETIN 737, U. S. DEPARTMENT OF AGRICULTURE. 



side of leaf tobacco, (<••) eggs under glass cover. A check was kept for 

 each experiment. It was found that with eggs exposed directly to the 

 rays, without covering as under arrangement (a), at a distance of 

 about 8 inches from the quartz mercury arc, an exposure even as short 

 as 30 seconds was effective in sterilizing eggs under two days old. As 

 has been found with Rontgen-ray radiation, the further embryonic 

 development has advanced, the more resistant the eggs become, and 

 longer exposures are required for their successful sterilization. Eggs 

 exposed on the underside of the leaf, arrangement (b), and those 

 under a cover of ordinary glass, arrangement (c), hatched normally. 

 Ordinary glass has the property of transmitting ultra-violet rays only 

 to a slight degree, its transmission being approximately onty one one- 

 thousandth that of quartz glass. It was found that a screen over the 

 eggs, even of the thinnest glass such as is used for cover glasses on 

 microscope slides, was sufficient to protect them completely from the 

 effect of ultra-violet rays. These exposures served as an additional 

 check on the results obtained from direct exposures to the rays. The 

 experiments with the eggs on the underside of the leaf showed that 

 the ultra-violet ra.ys would not penetrate even the thinnest leaf of 

 cured tobacco. This fact makes it impracticable to apply the method 

 in treating infested tobacco. 



Although eggs of the tobacco beetle when exposed directly to the 

 ultra-violet rays can be quickly and effectively sterilized, experiments 

 made so far do not indicate that other stages of the beetle are de- 

 stroyed by the same exposure. Treated larvae and pupae completed 

 transformations to the adult stage, and treated adults laid fertile 

 eggs and died off at a normal rate. The effect of more intense radia- 

 tion or of prolonged exposure of adults, pupae, and larvae has not 

 been determined. 



In order successfully to treat leaf tobacco containing eggs, it has 

 been suggested that in preparing the leaf for use it might be pos- 

 sible to devise means for smoothing out the leaves and thus make- 

 more powerful exposures of ultra-violet rays from all sides while 

 the tobacco passes through an exposure chamber containing the 

 lamps. As it would be extremely difficult, or perhaps impossible, by 

 any mechanical means, to smooth out completely all the creases or 

 wrinkles in the leaf tobacco, and as the rays do not penetrate opaque 

 substances like tobacco, the method, in the opinion of the writer, 

 is not likely to prove entirely effective or practicable, as the eggs of 

 the tobacco beetle ordinarily are deposited in wrinkles of the leaf. 

 The results obtained from several of the experiments with ultra- 

 violet rays are briefly summarized in Table VII. 



